Inductively coupled sources are becoming widely used for processing systems used in semiconductor manufacturing. Typical ICP sources employ an antenna that couples RF energy into a working, or processing, gas in a vacuum chamber, thus exciting a plasma in the gas. Such sources further employ an electrically insulating window or other electrically insulating material barrier between the antenna and the processing zone. A window, where used, may provide a barrier between atmospheric air and the vacuum of the chamber. Early use of high frequency coils for ionization of evaporated particles of coating material is described in U.S. Pat. No. 3,974,059 and 5,178,739.
In ICP semiconductor processing systems, the ICP source is an integral part of the vacuum chamber that contains the working or processing gas that is used for processing of the semiconductor wafers. For metal deposition and etching applications, a dielectric window or other electrically insulating structure has to be protected from plasma to avoid building-up conductive coatings on the surface of the insulating material that could prevent efficient RE power delivery into the plasma. Surface protection of the insulating material is provided by a structural device, namely, a deposition baffle placed between the plasma and the insulating material. The electrically insulating material is referred to hereafter as a window. Such a window is typically formed of a dielectric material such as ceramic. Deposition baffles made of slotted shields are described in U.S. Pat. Nos. 4,431,901; 4,795,879; 4,897,579; 5,231,334; 5,234,529; 5,449,433 and 5,534,231. Their use in ionized physical vapor deposition (iPVD) systems is described in U.S. Pat. Nos. 5,800,688 and 5,948,215, using cylindrical sources, and in U.S. Pat. Nos. 6,080,287 and 6,287,435 using planar flat and three-dimensional antennae.
A deposition baffle device or shield in a plasma processing system serves several purposes. Such a shield can provide protection, from plasma radiation, contamination and sputtering, for a dielectric window when the antenna is placed at the atmospheric side of the window, and for the antenna itself when the antenna is placed in the vacuum. In a metal deposition apparatus, such a shield may prevent the deposition of a conductive coating onto the surface of the dielectric window. The baffle or shield device is generally preferred to be opaque to the electrostatic fields but transparent to the electromagnetic fields, so that the device prevents electrostatic coupling of RF energy from the antenna to the plasma but allows magnetic coupling of the energy for the excitation of plasma. From the coupling efficiency standpoint, it is desirable to minimize the image currents on the shield so that energy is not wasted in joule heating of the shield.
Typically, a single design for a deposition baffle or shield cannot fully optimize all these aspects at once and so involves many trade-offs of these various requirements. For example, a shield that produces the least loss and is most transparent to electromagnetic fields is no shield at all. On the other hand, a perfect electrostatic and particle shield would entail complete enclosure of the antenna or dielectric window within a grounded case to separate it from the plasma environment, allowing no coupling at all. Optimization of a shield design is even more difficult when utilizing an antenna with a complex shape and structure rather than one in the form of a simple RF strip.
A uniform spatial coupling efficiency is desirable for a deposition baffle, particularly in plasmas used for processing of large diameter semiconductor wafers, because of a need for symmetric (at least, azimuthally uniform) RF power coupling into the plasma inside the chamber. Non-symmetric plasma tends to produce more contamination and erosion of hardware parts near the plasma source, including producing an irregular target sputter etch rate due to azimuthally non-uniform ion flux, which thereby produces a non-uniform etch or deposition. Usually, overall dimensions of a processing apparatus must be limited to several tens of centimeters, as there are requirements or at least preferences to keep a small footprint for the processing tool. Size limitations prevent enough space for using large components to keep the ends, edges and other irregular structural features away from critical locations.
For example, hot spots have been observed in deposition baffles close to the ends of slots, producing a locally more intense plasma, and as a result, locally enhanced erosion of the target has occurred. Non-uniform target erosion shortens the life of a target and thus increases the cost of ownership of the tool. Moreover, a non-uniform erosion rate may produce an oval or other irregular pattern in the film deposited on the substrate formed by a varying thickness of the deposited film on the substrate.
An important property of a deposition baffle is its transparency to electromagnetic fields. Slots allow azimuthal magnetic flux, which is produced by currents flowing in the conductors of an antenna that encircle the conductors in planes normal to the conductors, to pass through the baffle. An electric field is induced across the gaps between adjacent slots of the baffle that border the slots, which is in a direction such that it supports E×B movement of flux from the gap and away from the antenna. The transmission coefficient may reach values up to the 0.8-0.9 range. An electrically conductive deposition baffle, however, can produce two adverse effects on antenna-to-plasma coupling properties: (1) magnetic shielding of the antenna current Ia, and (2) possible significant ohmic losses. Both effects are stronger when magnetic flux normal to the surface o the baffle is increased.
Electrostatic shielding provided by a deposition baffle between the coil and the processing zone in the chamber makes it difficult to ignite plasma in an ICP reactor, especially at low pressures. In such case new procedures have to be developed to provide plasma ignition that is safe for the operating personnel, will not damage the hardware and will not interfere with the process or damage the substrates being processed.
The initial ionization of gas in the chamber requires a high enough voltage to cause electron and ion generation from neutral atoms. Further, to maintain the plasma, at least as many atoms hay to be ionized to produce ions and electrons as are lost by collisions within the chamber space or with the chamber walls. If too many electrons are lost, the plasma collapses or is never formed. A well-designed deposition baffle shields most of the electric field from the antenna and makes it difficult to ignite a plasma by an electric field ram the antenna. Increasing the RF current through the antenna to produce strange electric fields in its vicinity can result in high voltage at the antenna that can produce a atmospheric discharge outside of the chamber, and thus unsafe operation and potential component damage. Further, the lower the pressure in the chamber, the more difficult is plasma ignition.
In ICP plasma sources with slotted shields, increasing the opening of one of the slots in the shield can improve ignition, but this is not practical where dielectric window contamination is to be avoided. In systems having a target, it is possible to strike a plasma by DC power applied to the target. However, this may produce significant damage to the wafer being processed due to a high voltage spike at a powered electrode that can lead to unstable oscillations in the form of very large voltage perturbations that would continue until a threshold voltage is achieved and overcome, at which point full transition to the plasma would occur.